Genetic information is encoded on double-stranded deoxyribonucleic acid ("DNA" or "genes") according to the order in which the DNA coding strand presents the characteristic bases of its repeating nucleotide components. "Expression" of the encoded information to form polypeptides involves a two-part process. According to the dictates of certain control regions ("regulons") in the gene, RNA polymerase may be caused to move along the coding strand, forming messenger RNA (ribonucleic acid) in a process called "transcription." In a subsequent "translation" step the cell's ribosomes in conjunction with transfer RNA convert the mRNA "message" into polypeptides. Included in the information mRNA transcribes from DNA are signals for the start and termination of ribosomal translation, as well as the identity and sequence of the amino acids which make up the polypeptide. The DNA coding strand comprises long sequences of nucleotide triplets called "codons" because the characteristic bases of the nucleotides in each triplet or codon encode specific bits of information. For example, 3 nucleotides read as ATG (adenine-thymine-guanine) result in an mRNA signal interpreted as "start translation", while termination codons TAG, TAA and TGA are interpreted "stop translation". Between the start and stop codons lie the so-called structural gene, whose codons define the amino acid sequence ultimately translated. That definition proceeds according to the well-established "genetic code" (e.g., J. D. Watson, Molecular Biology of the Gene W. A. Benjamin Inc., N. Y., 3rd ed. 1976) which describes the codons for the various amino acids. The genetic code is degenerate in the sense that different codons may yield the same amino acid, but precise in that for each amino acid there are one or more codons for it and no other. Thus, for example, all of the codons TTT, TTC, TTA and TTG, when read as such, encode for serine and no other amino acid. During translation the proper reading phase or reading frame must be maintained. Consider for example what happens when the ribosome reads different bases as the beginning of a codon (underlined) in the sequence . . . GCTGGTTGTAAG . . . : EQU . . . GCT GGT TGT AAG . . . .fwdarw. . . . Ala-Gly-Cys-Lys . . . EQU . . . G CTG GTT GTA AG . . . .fwdarw. . . . Leu-Val-Leu . . . EQU . . . GC TGG TTG TAA A . . . .fwdarw. . . . Trp-Leu-(STOP).
The polypeptide ultimately produced, then, depends vitally upon the spatial relationship of the structural gene with respect to the regulon.
A clearer understanding of the process of genetic expression will emerge once certain components of genes are defined: